1. Technical Field
The present invention relates to methods and apparatus for sound amplification and public address systems utilizing no electricity or mechanical moving parts. In particular, the invention pertains to fluidic acoustic signal amplification.
2. Discussion of the Prior Art
Amplification, processing and transmission of sound has been the object of many inventions in the past, starting with Thomas Edison. Most of these inventions dealt with mechanical, electro-mechanical, and electronic reproduction of the sound signal by converting mechanical vibrations into electrical impulses, in the simplest form, merely coupling the mechanical vibrations directly to resonating boards and the like. Amplification and reproduction of sound by non-electronic, fluidic means was first suggested in U.S. Pat. No. 3,425,430 (Horton). Other patents, such as U.S. Pat. Nos. 3,239,027 (Schuck), 3,666,273 (Kantola et al), 3,398,758 (Unfried), 3,999,625 (Pickett), 4,121,620 (Pickett), 4,258,754 (Pickett) and 4,373,553 (Drzewiecki), also disclose fluidic sound or acoustic amplification systems of one sort or another. However, none or these consider or suggest the difficulty of fluidically processing audio signals in such a way that there is true representation and fidelity of the original signal, both in the amplification system and after the signal has been broadcast to a remote location. This can be seen in the early work by Roffman and Deadwyler on public address systems demonstrated by Horton at the 10.sup.th Anniversary of Fluidics Symposium at Georgia Tech in 1969. This system, while demonstrating a capability to amplify sound, was limited in bandwidth and produced significant amounts of hiss, noise and distortion.
In many instances it is desirable to amplify, process, transmit and broadcast sound from a single source to a number of receivers or listeners without using electricity, without closing an electrical circuit that would cause the flow of electricity, and without causing or having caused the movement, deflection or distortion of a mechanical member, such as a diaphragm, that might generate heat by friction. Among such instances, but certainly not the only ones, are public address systems in environments wherein heat or spark emissions from electrical elements or moving mechanical parts could cause a fire or explosion; e.g., environments where fuel fumes could easily be ignited, chemical explosive manufacturing plants, oil refineries, paint factories, plastics manufacturing plants, and grain mills where explosions from the extremely rapid combustion of dust and powder are a constant danger. Other instances where it is undesirable to have any electrical or electronic components include fail-safe operation of an audio system in environments threatened by interference from electromagnetic or nuclear radiation. Further, at facilities where radiation hazards exist, it is often difficult and very expensive to harden electronics sufficiently to withstand existing radiation levels. A non-electronic system that is inherently immune to radiation would greatly improve the safety of operations at such facilities. There are other instances where electricity simply cannot be used at all because of moral or religious teachings. In Judaism, strict orthodox interpretations of the Talmud proscribe the use of electricity on the Sabbath and Holy Days. The Amish have similar prohibitions against electricity in their daily lives. In places of worship where large congregations must be accommodated, communicants performing religious rites often cannot be heard in the far reaches of the building without excessive efforts resulting in a strain on their voices. Elderly and hearing impaired congregants in such situations can be denied active participation in their religious obligations.
Although fluidic amplification of sound is well-known, there is no practical fluidic system capable of providing reasonably good to high fidelity with adequate sound level to project the sound into large spaces in a manner comparable to conventional electronic devices. Hiss and background noise can be eliminated by using second generation laminar fluidic amplifiers, the laminar proportional amplifier, LPA, as described by Drzewiecki in an article "The Fluidic Audio Intercom" published by the American Society of Mechanical Engineers in its Proceedings of the Twentieth Anniversary of Fluidics Symposium, Chicago, Ill., 1980. This device can provide essentially noiseless, distortionless amplification of sound up to frequencies of about 5000 Hz. However, unlike the response of a microphone diaphragm directly deflected by the sound pressure wave, sound impinging on the input port of a fluidic amplifier creates a flow that interacts with and deflects a laminar stream. In terms of pressure (i.e., loudness) amplification, this is not very efficient due to the low pressures required to maintain laminar, noiseless flow. After the sound is amplified and coupled to the atmosphere (i.e., broadcast), sound pressure losses occur when the high level sound over a small exit area is issued from a much larger area, as out of an exponential horn.
It is clear that to provide effective fluidic sound amplification a great deal of gain is necessary, much more than if the pressure were directly amplified. The total gain needed must be sufficient to raise the original sound level plus the amount needed to overcome the input and output losses. Gain in excess of 100 (40 dB) requires three or more stages of fluidic amplification. This gives rise to a further problem, the inherent amplification of the DC null offsets needed to correct the inevitable imperfections and asymmetries in the fluidic elements. A one-percent offset is considered good even for high precision manufacturing techniques, but such offset in the initial stage will saturate the third stage when the gain in the two succeeding stages is 50 or more (assuming a maximum recovered output, saturation, of 50 percent).